Crack kinematics prediction in concrete structures and assessment of their criticality

The observation of cracks in normally functioning reinforced concrete (RC) structures is expected as the tensile strength of concrete is relatively low. However, one or more of those cracks can start to propagate with increasing crack openings, localizing strains and potentially indicating structural safety shortcomings. In the present project, RC members with low amounts of transverse reinforcements subjected to shear forces have been studied with the help of digital image correlation (DIC). In such structures, a critical shear crack (CSC) typically develops rapidly and can cause brittle failures. Assessing the criticality of such a crack could be a useful tool for engineers to prevent unpredictable failures that can be deadly. The present study is in continuation of Hugo Nick’s Master thesis (2023) [17] where the first avenue of a method to predict crack kinematics evolution from a given initial state was proposed. Obtaining relia-ble predictions of the failure crack kinematics in RC members with shear reinforcements proved to be challenging. Indeed, several hypotheses valid for RC elements without shear reinforcement must be questioned even for elements with low shear reinforcement ratios, notably the rigid-body hypothesis. The presence of stirrups and secondary cracks connected to the CSC strongly influence its kinematics and create a complicated deformation scheme with local flexion. Only approximative predictions have been achieved in the present study and the crack kinematics prediction question remains open. Knowing the crack kinematics allows to compute the shear transfer actions (STA). Identifying the load level at which the sum of the STA start decaying as the actual shear force continues to augment could indicate an unstable crack propagation and the structure’s imminent failure. A decrease of the total STA was indeed observed around 90% of the ultimate load for the specimens with a shear reinforcement ratio 𝜌𝑤>0.1%, but not on the other specimens. Individually studying the STA revealed a high sensitiv-ity of the residual tensile strength of concrete to the minimal crack width considered, and by extension the DIC error. It was also the case to a lesser extent for aggregate interlock. This parameter should be carefully selected. Even though the individual STA show variability across load levels and specimens, the statistical analysis of the sum of the STA for all SM10 specimens demonstrates that it follows the applied load relatively closely. An attempt at understanding the CSC propagation as a function of the load level was also conducted and lead to the discovery of a load-propagation experimental relationship depending on the position of the failure crack tip at an initial load level and approximately proportional to the square of the shear rein-forcement ratio. This relation offers an estimation that captures the general load-propagation trend despite the variability on the actual crack propagation. It was applicable on all tested specimens but could be limited to their specific experimental conditions. An attempt to predict the ultimate load has also been conducted using this load-propagation empirical relationship. A common way of determining whether a RC structure is close to failure or not is to compare the acting loads with the strength obtained from nonlinear finite element analysis (NLFEA) . However, its applica-tion on RC elements with localised cracking, such as low transverse reinforcement ratio beams subject-ed to shear can lead to unrealistically high results. Mesh sensitivity and localisation issues were identi-fied in the tested models. It is therefore important to study the limitations of NLFEA in such cases that differ from the habitual scope in order to avoid misinterpretation of the results.